1. Field of the Invention
This invention relates to radar systems generally and more specifically to frequency modulated pulse radars employing pulse compression.
2. Description of the Prior Art
In the prior art, frequency-modulation pulse compression systems are known. The text "Introduction to Radar Systems" by Merrill I. Skolnick (McGraw Hill 1962, Library of Congress Catalog No. 61-17675) provides background information, particularly in Section 10.9 of that text. Additional technical literature references are called out therein for further background information relating to the general type of system to which the present invention applies.
According to the published art, it is old to frequency modulate the transmitted pulse of a radar system and to use pulse compression on the received pulse. Such pulse compression has been effected through use of "matched filters," dispersive delay lines, or the like. Also known is step-wise frequency modulation approximating a linear or other frequency modulation function in discrete steps within the transmitted pulse. Through the use of a number of frequency steps within each transmitted pulse, a smooth analog curve of frequency variation can be closely approximated, and the system is, in view of the discrete step arrangement, readily adapted to digital implementation.
The so-called Bragg cell, which is a key element of the combination of the invention is known per se and has been employed in spectrum analysis and simultaneous frequency monitoring over a wideband of frequencies. The technical literature describes and documents the characteristics of Bragg cells as such. Their implementation in various applications of interest is also documented. One such technical report is entitled "Acousto-Optics Light the Path to Broadband ESM Receiver Design" which appeared in the periodical "Microwaves," September 1977. That periodical is published by Hayden Publishing Company, Rochelle Park, N.J.
Another summary article concerning Bragg cells appeared in the July/August 1976 issue of EW (Electronic Warfare), a periodical published by E. W. Communications, Inc., 3975 East Bayshore Road, Palo Alto, Calif. Suffice it to say herein that those of skill in this art will be able to construct a Bragg cell component and provide its supporting circuitry as required in the combination of the present invention.
Basically, the Bragg cell is a device capable of deflecting a light beam directly as a function of an applied frequency signal, and can operate with a frequency control signal in the GHz region providing the capability of handling directly received (and suitably power amplified) radar echo signals. The Bragg cell may also, of course, be applied at lower intermediate frequencies.
In the prior art, the transmitted pulse frequency-modulation envelope is normally generated by a first apparatus, and the received echo signals are correlated by a second apparatus including the matched filter or the dispersive delay line technique, for example. Whether the system is of the analog or digitally instrumented type, the fact of separate transmitting and receiving frequency modulation and pulse compression apparatus, respectively has been a serious problem in systems of that type. Time-frequency tracking between the transmitting and receiving frequency variation functions is vital if good performance from such a system is to be achieved. In linear pulse FM systems for example, it has been difficult to match the transmitting FM function with the received correlation function, and to achieve the type of stability required.
The manner in which the present invention deals with the disadvantages of the prior art to provide a unique pulse FM radar system with stable matching of the received signal correlator will be understood as this description proceeds.